Process for converting phthalic acid into terephthalic acid



PROCESS FUR CGNVERTING PHTHALIC ACID INTU TEREPHTHMKC ACKD Bernhard Raecke, Dusseldorf, Germany, assignor to Hcnlrel & Cie. G. m. b. EFL, Dnsseldorf=ll-lolthausen, Germany, a corporation of Germany No Drawing. Application January 24, 1957 Serial No. 635,945

Claims priority, application Germany November 21, 1952 14 Claims. (Cl. 260-5l5) This invention relates to a process of producing terephthalic acid, and more particularly to a process of producing terephthalic acid by isomerization of phthalic acid, isophthalic acid and other aromatic polycarboxylic acids.

This application is a continuation-in-part of my copending applications Serial No. 392,512, filed November 16, 1953, now abandoned; Serial No. 449,266, filed August 11, 1954, now abandoned; and Serial No. 472,245, filed November 30, 1954, now abandoned.

The known processes of producing terephthalic acid are based upon a conversion into carboxylic groups of reactive substituents in the 1,4 position of benzene. In practical operation this procedure consists in oxidizing the methyl groups of p-xylene to carboxyl groups. These processes are expensive in starting materials, operation and yield and often produce high yield of undesired isomers.

I have discovered that terephthalic acid can be produced by subjecting alkali metal salts of benzene polycarboxylic acids, in Which the carboxyl groups stand in orthoor meta-position relative to each other, to a heat treatment at a temperature of above about 340 C. and below the decomposition temperature of the starting materials and reaction products, preferably between about 340 and 500 C., whereby the alkali metal salts are transformed to a large extent into the corresponding alkali metal salts of terephthalic acid. Under certain conditions, particularly when the sodium and rubidium salts are used, the heat treatment also results in the formation of alkali metal salts of other valuable benzene carboxylic acids, such as benzoic acid and trimesitinic acid, which can be used for further transformation reactions, as described in application Serial No. 605,702, filed August 23, 1956, now U. S. Patent No. 2,823,230, and Serial No. 632,030, filed January 2, 1957, to produce terephthalic acid or other benzene carboxylic acids.

It is one object of this invention to provide a process of producing terephthalic acid and its derivatives by using as starting materials phthalic acid, isophthalic acid and other benzene polycarboxylic acids which are readily available in large amounts and which are relatively inexpensive.

Another object of this invention is to provide a simple and effective process of converting phthalic acid, isophthalic acid and other benzene polycarboxylic acids into terephthalic acid by isomerization.

Various other objects and advantages of my invention will appear as this description proceeds.

I have discovered that the alkali metal salts of phthalic acid and especially the potassium, sodium, lithium, ru bidium and cesium salts of phthalic acid, isophthalic acid and other benzene polycarboxylic acids can be rearranged to the alkali metal salts of terephthalic acid. In general I prefer to use the dialkali metal salts of these acids, but the mono-alkali metal salts may also be used. This rearrangement is efiected by heating the alkali metal salts of phthalic, isophthalic and other benzene poly- ICC carboxylic acids to higher temperatures. Very good yields are obtained when using dipotassium phthalate. Said salt and other alkali metal salts can be employed as such in this reaction. It is, however, also possible to start with reaction mixtures which yield alkali metal salts of phthalic acid, isophthalic acid and other benzene polycarboxylic acids instead of starting with the finished dialkali metal salt of the starting acid. For example, mixtures of phthalic acid anhydride and potassium carbonate are especially suitable. The reactants need not be present in stoichiometric proportions. If desired, the one or the other component may be used in excess.

In general, a reaction temperature above 340 C. is required to achieve a speed of reaction suflicient for technically attractive yields. The decomposition temperature of the organic substances and reaction products formed determines the upper limits of the reaction temperature which may be used. In general, a reaction temperature of 500 C. should not be exceeded for any long period because otherwise too much carbonization takes place in the starting materials and reaction products.

It is also necessary'to exclude any action of oxygen upon the organic reacting material at said high temperatures. Therefore, the rearrangement should be carried out in an inert atmosphere. It has been found to be quite advantageous to operate in the presence of carbon dioxide because better yields are thereby obtained; however, other inert gases may be used, such as nitrogen, carbon monoxide, ammonia gas, methane, ethane, propane, benzene, other hydrocarbon gases, noble gases and the like.

The starting materials to be rearranged should be uniformly heated and for this reason it is desirable to thoroughly agitate the reacting mass, for instance, by stirring or agitation. Preferably autoclaves provided with stirring devices are used.

Other ways of carrying out the reaction in a technically simple manner consist in heating the alkali metal salts of phthalic acid, isophthalic acid and the like or mixtures of compounds forming said salts Without agitation in a thin layer in the presence of carbon dioxide or other inert gases. For this purpose the space in the reaction apparatus may be sub-divided by trays or partitions into narrow reaction chambers. The trays or partitions may be made of metal and are preferablyarrang'ed at short distances from each other, for instance, at distances of a few centimeters.

An especially advantageous embodiment of such an apparatus consists of a replaceable insert which is provided with a jacket and with partition walls of steel or sheet iron arranged at a distance of 1 to 3 cm. and which can be inserted in and removed from a pressure-tight reaction chamber which can be heated to the required temperature. The partition walls or trays and also the outer Walls of said insert may be provided with holes. Said insert maybe filled by simply pouring the dry dialkali metal salts or mixtures forming said salts, such as phthalic or isophthalic acid and an alkali metal carbonate thereinto. The reaction product can'be readily dissolved out of said insert.

The presence of water during reaction must be avoided because even small amounts of moisture decreases the yield.

It is not necessary to use pure alkali metal phthalates salts, for instance, impair the yield on reaction, whereas mercury salts have a favorable influence.

The recovery of the terephthalic acid from the reaction mass is relatively simple, due to the different solubilitics of phthalic acid and terephthalic acid in water. In general, the procedure consists in dissolving the reaction mixture consisting of the alkali metal salts in water, filtering oil impurities, and precipitating the organic acids from the resulting solution by adding mineral acids such as hydrochloric acid or sulfuric acid. Unrcacted starting materials such as phthalic acid, for example, and any other water-soluble benzene carboxylic acids which may have been formed during the rearrangement reaction can be separated from the resulting acid mixture by extraction with water, whereby terephthalic acid remains as insoluble residue. Said acid can readily be obtained in a pure state by dissolving the same in alkali hydroxide solution and reprccipitation with mineral acids. The dipotnssium salt or other alkali metal salts obtained by such rearrangement can be used directly for producing derivatives of terephthalic acid. Said salts, for instance, can be converted into the dichloride or into esters, according to known methods. The recovered phthalic acid or isophthalic acid can again be used for reconversion into terephthalic acid. for instance, after converting it into the di-potassiurn salt or other alkali metal salt. Gthcr benzene carboxylic acids. for instance, benzoic acid and trimesic acid can be recovered as by-products from the mother liquors. and can be converted into alkali metal salts and used in the transformation or rearrangement reaction herein described.

The preferred (ii-potassium salt of phthalic acid or other alkali metal salts of phthalic, isophthalie or other aromatic polycarboxylic acids used as starting materials may be obtained by neutralization of the starting acid, such as phthalic acid or phthalic acid anhydride, iso phthalic acid or the like. in aqueous solution with potassium hydroxide, potassium carbonate, sodium hydroxide, sodium carbonate or other monovalent alkali metal bases. Complete neutralization is not necessary in all instances and some mono-alkali metal salts of the starting acid or acid mixture may be present. The solutions are dried on heated drum driers or by spray-drying. Thereby very finely divided powders are obtained which have only a very small moisture content and are well suited for carrying out the rearrangement according to this invention.

The following examples serve to illustrate this invention without. however. limiting the same thereto. Autoclaves provided with stirring devices and having a useful capacity of 500 to 1500 cc., if not otherwise stated, were used. The stirring devices consisted of a high-grade steel stirrer. the autoclaves were lined on the inside with a replaceable. high-grade steel lining and were heated electrically.

Where di-potassium phthalate was used as starting material in these examples it was obtained in the following manner:

148 g. (1 mol) of pure phthalic acid anhydride were neutralized in 1.5 liters of water with 112 (2 mol) of pure 100% potassium hydroxide dissolved in Water. The resulting solution was spray-dried in a Krause spraydrying apparatus with hot air of a temperature of 110 C. The powder was kept over night in a drying oven at a temperature of 120 C. and was immediately thereafter filled into tightly-closed glass bottles because it is hygroscopic. The sodium, lithium, rubidium and cesium salts may be produced in a similar manner or may be formed in situ in the reaction chamber by reaction of the sta1ting aromatic carboxylic acid and sodium carbonate, lithium carbonate and the like.

In all instances the purity of the terephthalic acid re sulting from the reactions was determined by conversion into the dimethyl ester and by determining the melting point of said ester (140.8 C.). The dimethyl ester, in general, is produced by first converting the acid by means of phosphorus pentachloride into the dichloride which is then reacted with methanol. Almost theoretical yields of the dimethyl ester are obtained in this manner. in some instances the dimethyl ester was also produced by csterifying the free acid by means of methanol and gaseous hydrochloric acid. The dimethyi ester can reatlily be sublimcd and has a melting point of 140.55 C.

I had determined by preliminary cxpei' car; that it mixture of phthalic acid and terephthalic acid of 11am. :1 constitution could be substantially completely s 'u'atccl into its components by treatment with boiling wa! parts of phthalic acid are soluble in parts of n 0 water. Traces of phthalic acid which might remain in the terephthalic acid can be sublimed on drying; at a temperature of C. in the form of phthalic acid anhydride. It is, therefore, possible to cll'cct separation of the acids by removing phthalic acid by sublimation in the form of its anhydride. Similarly isophtha ic acid is soluble in hot water to the extent of 1 part per #60 parts of hot water, while terephthalic acid is, for all practical purposes, insoluble in either hot or cold water. The above-described methods of working up the reaction product was employed in all examples.

Example I 50 g. of di-potassium phthalate are heated in an auto clave provided with stirrer to a temperature of 350 354 C. for six hours. Heating to said temperature requires one hour. Carbon dioxide is tulpplicd under pressure to the autoclave at the beginning of the experiment. The initial pressure in the cold is 30 atmospheres gauge. The highest pressure at the reaction temperature i i atmospheres gauge. After cooling, the salt mixture is dissolved in 200 cc. of hot water. the solution is filtered. whereby darkly-colored impurities remain. and the organie acids are precipitated by the addition of dilute 1ndrochloric acid. The precipitated mixture of the resulting acids is filtered by suction on a porcelain funnel. and is washed with a small amount of cold water. The iiltered product is then boiled with 200 cc. of water and again filtered by suction. After wasl ng with boiling water. there remains a white, water-insoluble residue which is dried at 120 C. for fourteen hours. 2.85 g. of acid are obtained which proved to be pure tercphthalic acid. The yield amounts to 8.3% of the acid used. On cooling most of the unreacted phthalic acid cry. aili/cs from the aqueous filtrate and can again be used in new rearrangement reactions.

Example 2 A mixture of 80 g. of di-potassium phthalate with 20 g. of potassium carbonate is treated in an autoclave in the same manner as described in Example 1 at a temperature of 350-354 C. for six hours. Heating of the reaction mixture to said temperature requires 1 hours. The initial pressure of carbon dioxide is 30 atmospheres gauge; the highest pressure reached during th.: reaction, 82 atmospheres gauge. The reaction mixture is separated as described in Example 1, whereby, however, 300 cc. of water are used in place of 200 cc. The unreaeted phthalic acid is completely dissolved thereby. 5.8 g. of terephthalic acid, corresponding to a yield o 10.7% of the acid subjected to this rearrangement rcaction, are obtained. Unreacted phthalic acid is recovered by crystallization from the aqueous extract.

Replacing carbon dioxide with hydrogen or nitrogen under otherwise like pressure does not yield tercphthalic acid as a final product. Merely small amounts of a dart: smudgy water-insoluble residue remain.

Example 3 A mixture of 80 g. of di-potassium phthalate and 20 g. of potassium carbonate is treated at a temperature of 350-354 C. for 30 hours. Heating said mixture to said reaction temperature requires 1% hours. initial pressure of carbon dioxide supplied to the autoclave is 30 atmospheres gauge, and'the maximum pressure. reached during the reaction is 70 atmospheres gauge. I

14.8 g. of terephthalic acid corresponding to 27% of the acid subjected to said rearrangement reaction are obtained after working up the reaction mixture as described in Example 1.

The unreacted phthalic acid is recovered from the aqueous extract and is again used for such rearrangement reaction after conversion into the di-potassium salt.

Example 4 4 g. of mercury chloride are added to a mixture of 80 g. of di-potassium phthalate and 20 g. ofpotassiurn carbonate. The mixture is heated under carbon dioxide pressure to a temperature of 350354 C. for six hours. Heating said mixture to said temperature requires 1% hours. The initial pressure is 30 atmospheres gauge and the maximum pressure 72 atmospheres gauge. The resulting crude product which contains some mercury droplets is separated in the same manner as described in Example 1. 7.7 g. of terephthalic acid corresponding to 14% of the phthalic acid subjected to said rearrangement reaction are obtained.

When, in place of mercury chloride, the same amount of copper chloride is used, similar results are obtained. When using the same amount of nickel chloride, however, no terephthalic acid is produced.

Example 5 A mixture of 160 g. of di-potassium phthalate and 40 g. of potassium carbonate is filled into a copper tube and is heated to a temperature of 400425. C. for 7 /2 hours while stirring and passing carbon dioxide therethrough. Heating said reaction mixture to said temperature requires about one hour. The black residue is worked up in the same manner as described in Example 1. 12.5 g. of a dark, water-insoluble acid are obtained, said acid mostly consisting of terephthalic acid.

Example 6 150 g. of di-potassium phthalate containing 0.5% of water are heated in an autoclave provided with a stirring device to a temperature of 400402 C. for six hours. Heating the reaction mixture to said temperature requires about 2 /2 hours. Carbon dioxide is supplied to the autoclave under pressure. The initial pressure in the cold amounts to 30 atmospheres gauge and the maximum pressure on heating amounts to 75 atmospheres gauge. After cooling, the dark gray reaction product is worked up as described in Example 1. It is dissolved in 600 cc. of hot water and the solution is filtered. 5 g. of dark impurities remain on the filter. The organic acids are precipitated from the filtrateby the addition of dilute hydrochloric acid. The mixture of the resulting acids is filtered by suction on a porcelain funnel and is Washed with a small amount of cold water. In the moist state it represents a reddish, not distinctly crystallized mass. This product is boiled with 550 cc. of Water and the hot solution is filtered by suction. After washing with a small amount of boiling water, there remains a slightly pink, water-insoluble residue which is dried at 110 C. for 48 hours. 58 g. of terephthalic acid corresponding to 56.5% of the acid used in said rearrangement reaction are obtained.

No phthalic acid crystallizes from the mother liquors resulting from the acid precipitation and the water extraction.

Ex pl 7 150 g. of di-potassium phthalate mixed with 19 g. of potassium carbonate are heated in the same manner as described in Example 6 to a temperature of 400-404 C. for six hours. The initial pressure of the carbon dioxide is 30 atmospheres gauge and the maximum pressure is 70.5 atmospheres gauge. The reaction mixture is worked up in. the same manner as described in Example 6. 12.8 g. of a black slurry remain as filter residue.

52 g. of terephthalic acid corresponding to 50.8% of the acid used in this rearrangement reaction are obtained.

Example 8 150 g. of di-potassium phthalate are heated to a temperature of 450 C. for one hour, whereby the initial carbon dioxide pressure is 30 atmospheres gauge. On working up the reaction mixture in the same manner as described in the preceding examples, 48 g. of terephthalic acid, corresponding to 47% of the acid used in said rearrangement reaction are obtained.

Example 9 A mixture of 150 g. of di-potassium phthalate and 150 g. of potassium sulfate is heated in an autoclave to a temperature of 400-403 C. for six hours. The initial carbon dioxide pressure is 30 atmospheres gauge, and the maximum pressure 'is 79 atmospheres gauge. After working up the reaction mixture as described in the preceding examples, 57 g. of terephthalic acid, corresponding to a yield of- 55.4%, are obtained.

When using, under otherwise similar conditions, a mixture which contains in place of potassium sulfate the same amount of potassium chloride, the yield of terephthalic acid is 59 g., corresponding to a yield of 57.7%. The crude product has an intense benzene odor.

Example 11 150 g. of di-potassium phthalate are heated to a ternperature of 450 C. for six hours. Nitrogen is used as protective gas, whereby the initial pressure is 60 atmospheresgauge, and the maximum pressure is 184 atmospheres gauge.' On working up the reaction mixture as described in the preceding examples, 25.7 g. of a black filter residue are obtained. The yield of terephthalic acid is 37 g., corresponding to a yield of 36%.

Using hydrogen in place of nitrogen, under otherwise like conditions, does not produce any terephthalic acid, but an oily liquid in a large amount.

When using carbon monoxide, under otherwise like conditions, as protective gas, 28 g. of terephthalic acid, corresponding to a yield of 27.2%, are obtained. Said crude product also has an intense benzene odor. In this case larger amounts (52 g.) of Water-insoluble filter residues remain.

Example 12 g. of mono-potassium phthalate are heated in an autoclave, while stirring, at an initial carbon dioxide pressure of 60 atmospheres gauge to a temperature of 450 C. for six hours. The reaction product has a strong benzene odor and yields 11 g. of terephthalic acid after working up according to Example 1.

Example 13 g. of di-potassium phthalate are heated in an autoclave provided with a stirrer to a temperature of 400 C. for eight hours. At' the beginning carbon dioxide is introduced into the autoclave under a pressure of 5 atmospheres. On heating the reaction mixture to said reaction temperature, the pressure is allowed to increase to 10 atmospheres gauge and is then kept constant at 5 atmospheres gauge by allowing carbon dioxide to escape.

On working up the reaction product g. of terephthalic acid, corresponding to a yield of 29.5% are obtained.

Example 14 A mixture of 300 g. of di-potassium phthalate and 15 g. of potassium pyrophosphate is heatedin an autoclave while stirring, to a temperature of 400 C. for eight hours. At the beginning atmospheres gauge of carbon dioxide are introduced into the autoclave. The maximum pressure reached during the reaction is 151 atmospheres gauge.

After working up the reaction mixture according to the procedure of Example 1, 112 g. of pure terephthalic acid, corresponding to a yield of 54.4% are obtained.

Example 15 An autoclave is charged with a mixture of 148 g. of chemically pure phthalic acid anhydride and 140 g. of anhydrous potassium carbonate. Carbon dioxide is introduced under pressure into said autoclave, whereby the initial pressure is 50 atmospheres gauge. The autoclave is heated to a temperature of 400 C. for six hours, whereby a maximum pressure of 195 atmospheres gauge is reached. uct amounting to 230 g., which has a slight benzene odor, is dissolved into 500 cc. of water. The solution is heated to boiling and is filtered. The filter residue is again boiled with 300 cc. of water and is filtered. The combined solutions are acidified at boiling temperature with dilute hydrochloric acid. After cooling, the precipitating organic acids are filtered by suction on the suction filter. The residue is extracted twice, each time with 300 cc. of water, at boiling temperature. The remaining residue of almost white color is dried at 130 C. for six hours. g. or" chemically pure terephthalic acid are obtained.

3.5 g. of water-soluble benzene carboxylic acid are recovered from the mother liquors, so that the total yield of terephthalic acid is 65.4%.

Example 16 A mixture of 150 g. of technical grade phthalic acid anhydride and g. of anhydrous potassium carbonate is heated in an autoclave, while stirring, to a temperature of 400 C. for six hours. At the beginning of heating, carbon dioxide is introduced into the autoclave under a pressure of 50 atmospheres gauge. The maximum pressure reaches 219 atmospheres gauge. The resulting dark gray product, having a benzene odor, is worked up in the usual manner. 99 g. of pure tereph thalic acid, corresponding to a yield of 59.6%, are isolated.

Example 17 A mixture of 150 g. of technical grade phthalic acid anhydride and g. of potassium carbonate is heated in an autoclave to a temperature of 400 C. for seven hours. At the beginning of the heating, carbon dioxide is introduced into the autoclave at a pressure of 50 atmosphercs gauge. The maximum pressure reaches 250 atmospheres gauge.

The reaction product is worked up in the manner described in the preceding examples by boiling with water. 116 g. of terephthalic acid corresponding to a yield of 69.9% are obtained.

Example 18 A mixture of g. of technical grade phthalic acid anhydride and 140 g. of potassium carbonate is heated in an autoclave to a temperature of 400 C. for eight hours. At the beginning of the heating, carbon dioxide is introduced into the autoclave at a pressure of 15 atmospheres gauge. The maximum pressure reaches 68 atmospheres gauge.

On working up the reaction mixture according to the After cooling, the gray, solid reaction prodpreceding examples, 113 g. of terephthalic acid corresponding to a yield of 68.1% are isolated.

5 g. of a carboxylic acid mixture consisting primarily of benzoic acid and a small amount of trimesic acid are isolated from the mother liquor resulting from a precipitation of the acids. When working in the same manner, but at an initial carbon dioxide pressure of 35 atmospheres gauge and a maximum pressure of 108 atmospheres gauge, 119 g. of terephthalic acid, corresponding to a yield of 71.7%, are obtained. Working at an initial carbon dioxide pressure of 50 atmospheres gauge and a maximum 'pressure of atmospheres gauge yields 115 g. of terephthalic acid corresponding to a yield of 69.3%. 20 g. of an acid mixture are obtained from the mother liquors of said last mentioned experiment after evaporation to half their volume and allowing the concentrated mother liquors to stand at 0 C. Said acid mixture has been found to be a mixture of beuzoie acid and trimesic acid.

Example 19 A mixture of 150 g. of technical grade phthalic acid anhydride, 140 g. of potassium carbonate, and 100 g. of. well-dried coarse-grained sand is heated in an autoclave to a temperature of 400 C. for eight hours. At the beginning of the experiment carbon dioxide is introduced into the autoclave under a pressure of 50 atmospheres gauge. The maximum pressure amounts to 196 atmospheres gauge.

After cooling the resulting light-grayish product, which has a faint odor of benzene, is worked up in the manner described in the preceding examples. 124 g. of tcrephthalic acid, corresponding to a yield of 74.1%, are obtained.

The mother liquors resulting from the acid precipitation as well as from the extraction with hot water are concentrated by evaporation to half their volume and are cooled to 0 C. After allowing the concentrated mother liquors to stand for some time, 9 g. of an acid mixture precipitates, said acid mixture being a mixture of benzoic acid and trimesic acid.

Example 20 A mixture of 150 g. of technical grade phthalic acid anhydride, 125 g. of potassium carbonate and 50 g. of coke in small pieces of a diameter of 2-5 mm. is heated in an autoclave, while stirring, to a temperature of 400 C. for six hours. At the beginning of the experiment carbon dioxide is introduced into the autoclave at a pressure of 50 atmospheres gauge. The maximum pressure amounts to 192 atmospheres gauge.

After cooling, a light gray product is obtained, which is worked up in the manner described in the preceding examples. 102 g. of pure terephthalic acid corresponding to a yield of 61.4% are obtained.

Example 2] In the following example the autoclave was provided with an insert which permitted heating of the starting materials in thin layers. Said insert was made of highgrade steel. The reaction chamber was subdivided by said insert reaction zones by partition walls arranged at a distance of 1 cm. from each other. The partition walls were stiffened and held apart from each other by struts.

300 g. of di-potassium phthalate were filled into said insert and heated to a temperature of 400 C. for eight hours. At the beginning of the heating carbon dioxide was introduced into the autoclave under a pressure of 50 atmospheres gauge. The maximum pressure amounted to 149 atmospheres gauge. The reaction product was dissolved out of said insert and worked up according to Example 1. The gray, somewhat moist reaction product, having an odor of benzene, yielded 137 g. of terephthalic acid corresponding to a yield of 66.6%. When mixing the di-potassium salt with the same amount of sand (300 g.), the yield of terephthalic acid is 114 g. corresponding to a yield of 55.1%.

Exam le 22 The insert described in Example 21 is filled with a mixture of 150 g. of technical grade phthalic acid anhydride and 140 g. of potassium carbonate. A layer of 50 g. of pure potassium carbonate is additionally provided at the bottom of said insert, said potassium carbonate serving to absorb any molten phthalic acid anhydride emerging from the above-mentioned mixture. After inserting said insert into the autoclave, carbon dioxide is introduced thereinto under a pressure of 50 atmospheres gauge. The autoclave is heated to a. temperature of 400 C. for eight hours, whereby a maximum pressure of 202 atmospheres gauge is reached. After cooling the gray reaction product, having a faint odor of benzene, is worked up as described in Example 1.

33.5 g. of a black filter residue and 89 g. of terephthalic acid, corresponding to a yield of 53.6%, are obtained.

Example 23 A mixture of 150 g. of technical grade phthalic acid anhydride, 150 g. of potassium carbonate and g. of bo-ric acid anhydride is filled into an autoclave provided with an insert as described in Example 21 and heated to a temperature of 400 C. for eight hours. At the begin ning of the heating, carbon dioxide is introduced into said autoclave under a pressure of 50 atmospheres gauge. The maximum pressure amounts to 200 atmospheres gauge. 97 g. of terephthalic acid, corresponding to a yield of 58.5% are obtained.

When using g. of boric acid anhydride, 80 g. of terephthalic acid, corresponding to a yield of 48.2%, are obtained.

Example 24 150 g..of technical grade phthalic acid anhydride, 140 g. of potassium carbonate and 105 g. of finely pulverized sulfur are heated in an autoclave provided with a stirring device to a temperature of 400 C. for eight hours. At the beginning of the experiment carbon dioxide is introduced into an autoclave under a pressure of 50 atmospheres gauge. The maximum pressure amounts to 280 atmospheres gauge.

The reaction product exhibits an odor of hydrogen sulfide. 80 g. of terephthalic acid, corresponding to a yield of 48.2%, are obtained.

Example 25 A mixture of 150 g. of technical grade phthalic acid anhydride and 140 g. of potassium carbonate is heated with the addition of 2.8 g. of mercury in an autoclave provided with a stirring device to a temperature of 390 C. for eight hours. At the beginning of the experiment carbondioxide is introduced into the autoclave under a pressure of 50 atmospheres gauge. The maximum pressure amounts to 250 atmospheres gauge.

Working up of the reaction mixture yields 98 g. of pure terephthalic acid corresponding to a yield of 59.3%.

Example 26 A mixture of 150 g. of technical grade phthalic acid anhydride, 140 g. of potassium carbonate and 4 g. of copper oxide (CuO) is heated in an autoclave provided with a stirring device to a temperature of 400 C. for six hours. At the beginning of the experiment carbon dioxide is introduced into said autoclave under a pressure of 50 atmospheres gauge. The maximum pressure amounts to 178 atmospheres gauge.

Working up of the reaction mixture yields 95 g. of pure terephthalic acid corresponding to a yield of 56.5%.

Adding, in place of copper oxide, the same amount of manganese dioxide, 96g. of terephthalic acid, corresponding to a yield of 57.1%, are obtained; with the same amount of lead oxide, g. of terephthalic acid, corresponding to a yield of 59.4%, are recovered from the reaction mixture. Addition of 4 g. of iron oxide yields 63.6% of terephthalic acid. When using lead hydroxide Pb(OH) and proceeding otherwise under the same conditions, there are obtained 71.7% of terephthalic acid, while lead dioxide yields 67.7%, and 4 g. of litharge (PbO), yields 76.2% of terephthalic acid.

When using antimony oxide there are obtained 108 g. of terephthalic acid corresponding to a yield of 65.1%.

Addition of 4 g. of barium oxide causes a decrease in yield to 88 g. corresponding to a yield of 53%.

Example 27 A mixture of 150 g. of technical grade phthalic acid anhydride and 140 g. of potassium carbonate, as well as 150 g. of technical grade phthalic acid anhydride, 140 g. of potassium carbonate and 3 g. of iron oxide (Fe O are heated in an autoclave to a temperature of 400 C. for four hours. Carbon dioxide is introduced into the autoclave under a pressure of 5 atmospheres gauge. The maximum pressure reaches 60 atmospheres gauge.

Working up the reaction mixture yields 108 g. of pure terephthalic acid corresponding to a yield of 65.1%. When heating the reaction mixture for 3 hours only, the yield of terephthalic acid amounts to 104 g. correspondiug to a yield of 62.7%.

Example 29 A mixture of 150 g. of technical grade phthalic acid anhydride, 140 g. of potassium carbonate and 15 g. of chromium trioxide is heated in an autoclave provided with a stirring device to a temperature of 400 C. for six hours. The initial carbon dioxide pressure is 50 atmospheres gauge; the maximum pressure 151 atmospheres gauge. 73 g. of terephthalic acid, corresponding to a yield of 43.4% are obtained.

When using 10 g. of titanium dioxide in place of chromium trioxide, 108 g. of terephthalic acid, corresponding to a yield of 64.2%, are obtained, while when using 10 g. of vanadium pentoxide, 41.6% of terephthalic acid are produced.

Addition to 6 g. of Zirconium dioxide in place of the above-mentioned oxides yields g. of terephthalic acid corresponding to 63.3%.

Example 30 A mixture of 150 g. of technical grade phthalic acid anhydride, g. of potassium carbonate and 3 g. of tin dioxide is heated in an autoclave provided with a stirring device to a temperature of 400 C. for six hours. Carbon dioxide is introduced initially into the autoclave under a pressure of 50 atmospheres gauge. The maximum pressure amounts to 207 atmospheres gauge. 99 g. of pure terephthalic acid, corresponding to a yield of 58.9%, are obtained on working up the reaction mixture in the manner described in the preceding examples.

When adding in place of tin dioxide the same amount of arsenic trioxide, 60 g. of terephthalic acid, corresponding to a yield of 35.7%, are obtained, while arsenic pentoxide yields 87. g. of terephthalic acid corresponding to a yield of 51.7%.

Addition of 3 g. of tungsten trioxide in place of the 11 above-mentioned oxides yields 62 g. of terephthalic acid corresponding to a yield of 36.9%.

Example 31 Example 32 A mixture of 300 g. of di-potassium phthalate and g. of mill scale (Fe O is heated in a rotating drum to a temperature of 400 C. for four hours without pressure. Carbon dioxide is passed through the drum at the beginning of the run. Said passing through of carbon dioxide is repeated three times at intervals of about one hour. On working up the gray, crude product in the usual manner as described in the preceding examples, 39 of pure terephthalic acid are obtained corresponding to a yield of 18.9%. 51 g. of water-soluble benzene carboxylic acid crystallize from the mother liquor.

Example 33 The apparatus used consisted of an autoclave having a net capacity of 500 cc., which was made of high-grade steel. The autoclave contained a removable interior lining, also made of high-grade steel, and was electrically heated. An insert Was introduced into the autoclave which consisted of a series of vertical partitions spaced about 1 cm. apart; the vertical partitions were reinforced by horizontal braces which formed horizontal shelves. 150 g. of dipotassium isophthalate were evenly distributed over the horizontal shelves to form thin layers thereon. The autoclave was then closed and filled with carbon dioxide at atmospheres gauge. It was then heated to 400 C. for 8 hours, during which time the internal pressure reached 1.26 atmospheres gauge. The autoclave was then permitted to cool. The substance within the autoclave at the end of this period was very lightly colored and had an odor of benzene. It was removed from the autoclave, dissolved in Water, and the aqueous solution was filtered. A mixture of organic carboxylic acids was precipitated from the filtered solution by the addition of dilute hydrochloric acid. The precipitate was filtered off by suction and thoroughly extracted five times with boiling water. The remaining insoluble substance was found to be pure terephthalic acid. The yield was 43 g., which is 25.9% of the theoretical yield. 7 g. of bcnzoic acid were recovered from the hot water solution.

Example 34 In an autoclave as in Example 33, also equipped with an insert as described in that example, 300 g. of dipotassium isophthalate were spread in thin layers on the horizontal shelves and then treated as described in the preceding example. During the heating period the pressure in the autoclave reached 190 atmospheres gauge. The yield of terephthalic acid was 80 g, which is 48.2% of the theoretical yield.

Similar yields were also obtained when dipotassiun. isophthalate was isomerized to the terephthalate at 400 C. in the presence of carbon dioxide under pressure in an autoclave equipped with agitating means such as a stirrer instead of the above described insert.

By increasing the temperature during the heating period to 420 C.. the transformation from isophthalate to tcrcphthalate was completed after two or three hours. The yields were substantially the same as those mentioned earlier in this example.

Example 35 300 g. of dipotassium isophthalate, admixed with 10 g. mill scale (Fe O were charged into an autoclave equipped with an agitating device such as a stirrer. The autoclave was then closed and carbon dioxide was introduced until the pressure reached about 50 atmospheres gauge. Thereafter, the autoclave was heated to 400 C. for six hours while thoroughly agitating the contents. During this heating period the internal pressure increased to 188 atmospheres gauge. The autoclave was allowed to cool and the isomerized product was dissolved in water. The aqueous solution was then filtered and thereafter dilute hydrochloric acid was added to the solution, whereby a mixed precipitate of benzene carboxylic acids 3"15 formed. The precipitate was filtered off and thoroughly extracted with boiling water. The insoluble residue left behind was found to be pure terephthalic acid. The yield was 112 g., which is 54.4% of the theoretical yield.

Similar yields were obtained when, instead of adding mill scale, 6 g. zirconium dioxide, 8 g. tin oxide, 10 g. titanium dioxide, 3 g. pentavalcnt arsenic oxide, 3 g. tctravalent tin oxide or 7 g. lead oxide (litharge) are admixed with the dipotassium isophthalate.

Example 36 A mixture of 150 g. dipotassium isophthalate and 150 g. dipotassium phthalate, admixed with the metal oxides of Example 35, was charged into a rotary drum and subjected to an isomerization treatment as in the preceding example. The product was then dissolved in boiling water, filtered, and the free acids precipitated with dilute hydrochloric acid as described in Example 35. After separation with boiling water, the yield of terephthalic acid was 41 g., which is 20.4% of the theoretical yield. The extraction solution in boiling water yielded 46 g. of water-soluble benzene carboxylic acids.

Example 37 150 g. dipotassium isophthalatc and 7.5 g. of cadmium oxidekieselguhr catalyst were heated in a rotary auto clave having a net volume of 1.4 liters for 5 hours at a temperature of 430 C. Carbon dioxide was introduced into the autoclave before it was heated and the pressure of carbon dioxide in the cold was adjusted to 50 atmospheres gauge. Upon heating, the internal pressure reached a maximum of 150 atmospheres gauge. A reaction product weighing 146 g. was formed, g. of which were then boiled in water. The resulting solution was separated from the catalyst and the carbon formed during the transformation reaction by filtering the solution, and the filtrate was then heated and acidified with hydrochloric acid. The precipitated tercphthalic acid was twice extracted with boiling water and finally dried at C. The yield of pure terephthalic acid was 46.9 g., which is 66.4% of the theoretical yield. in order to show the purity of the terephthalic acid thus obtained. it was converted into the dimethyl ester. which was obtained with a yield of 97%.

Example 38 g. dry disodium phthalate were distributed in thin layers on the shelves formed by a lattice work of horizontal and vertical strips made of refined steel. The lattice work was then inserted into an autoclave having a net volume of 1000 cc., and the autoclave was closed. Thereafter, carbon dioxide was introduced into the auto clave to fiush out the air, and after the autoclave was rendered air-tight the introduction of carbon dioxide \va; continued until the pressure inside the autoclave reached 60 atmospheres gauge. The autoclave and its contents were then heated until the temperature in the interior reached 450 0; this temperature was maintained {or six hours. During this time the pressure in the autoclave reached a maximum of 148 atmospheres gauge. The conl3 tents were then allowed to cool to substantially room temperature. The autoclave was opened anda grayish black solid was removed from the shelves of the lattice work insert. The solid was dissolved in 500 cc. water, the solution was heated to boiling and immediately filtere Thereafter, the filtrate was acidified with hydrochloric acid until no more precipitate formed, and the precipitate was filtered oil by suction. The filter cake was repeatedly extracted with 200 cc. batches of hot water. 2.15 g. of a white solid remained which was insoluble in hot water; it was found to be pure terephthalic acid. The water used for extraction was collected and upon evaporation thereof a substantial quantity of benzoic acid crystallized out.

Example 39 A mixture of 140 g. technical grade phthalic acid anhydride, 116 g. anhydrous sodium carbonate and 6 g. ferric oxide (Fe O was placed into an autoclave made of refined steel having a net volume of 1500 cc. ,The autoclave was provided with a stirrer adapted to agitate the contents. The autoclave was then closed, the air was flushed out with carbon dioxide, and the autoclave was rendered air-tight. Thereafter, the introduction of carbon dioxide was continued until the pressure in the interior reached 50 atmospheres gauge. The contents of the autoclave were then heated to 400 C., and maintained at that temperature for about six hours. The pressure in the interior of the autoclave reached a maximum of 158 atmospheres gauge during that time. The contents were then allowed to cool to substantially room temperature. Upon opening the cooled autoclave, the contents were found to be a light gray substance which had an odor of benzene; it was dissolved in 1000 cc. water, and the resulting solution was heated to boiling and filtered while hot. Tie filtrate was acidified with hydrochloric acid until no more precipitate formed. Thereafter, the precipitate was filtered off by suction and repeatedly extracted with 500 cc. batches of boiling water. 8 g. of insoluble residue remained which was found to be pure terephthalic acid, which represents a yield of 4.8% of theory. The water used for extraction was collected and upon evaporation yielded 53 g. of a solid which was found to be a mixture of benzoic acid and trimesitinic acid.

Example 40 A mixture comprising 3.75 g. phthalic acid anhydride and 5.08 g. rubidium carbonate were placed into an autoclave having a net volume of 20 cc., and heated at 400 C.

for 5% hours in the absence of oxygen. The autoclave and its contents were then allowed to cool to substantially room temperature. The autoclave was then opened and its contents were found to be a grayish black solid substance. The substance was dissolved in 20 cc. hot Water, the resulting solution was filtered and finally aciditied with hydrochloric acid until no more precipitate formed. The acid solution and the precipitate were allowed to cool, whereupon the precipitate was filtered off. The filter cake was repeatedly extracted with 20 cc. batches of boiling water. An insoluble solid remained which weighed 0.68 g. and was analyzed to be pure terephthalic acid. This amount represents a yield of 17.7% of theory.

Example 41 100 g. of disodium phthalate admixed with 4 g. zinc dioxide were heated for 6 hours at 400 C., after introducing carbon dioxide to a pressure of 50 atmospheres gauge; the pressure rose to 121 atmospheres gauge. The reaction mixture was worked up as described in Example 1 and yielded terephthalic acid in an amount corresponding to 14% of the theoretical yield.

Example 42 150 phthalic acid anhydride, 75 g. lithium carbonate and g. cadmium oxide were heated in an atmosphere of carbon dioxide for 5 hours at a temperature of 400 C. in an autoclave. Before heating, the pressure of carbon dioxide was 47 atmospheres gauge and this pressure reached a maximum of 72 atmospheres gauge during the heating step. A black, solid reaction product weighing 155 g. was formed, which was subsequently treated as described in Example 1. The yield of pure terephthalic acid was 28 g., which is 17.2% of the theoretical yield.

Example 43 A mixture of 150 g. phthalic acid anhydride, 106 g. anhydrous sodium carbonate and 15 g. cadmium phthalate were heated for 6 hours at 400 C. in an autoclave in an atmosphere of carbon dioxide. The starting pressure of carbon dioxide was 50 atmospheres gauge and during the heating step the internal pressure reached a maximum of 180 atmospheres gauge. A dark gray substance weighing 202 g. was formed which, upon treatment as described in Example 1, yielded 32 g. terephthalic acid, which is 14.5% of the theoretical yield. The waste solutions yielded 14 g. water-soluble benzene carboxylic acids.

Example 44 g. anhydrous sodium carbonate, g. phthalic acid anhydride and 10 g. of a cadmium-kieselguhr catalyst were heated in a rotary autoclave in an atmosphere of carbon dioxide for 4 hours at a temperature of 440 C. Before heating, the pressure of carbon dioxide was 50 atmospheres gauge and, during heating, this pressure rose to a maximum of atmospheres gauge. A reaction product weighing 212 g. was formed, 100 g. of which were treated in the manner described in Example 1'. The yield of terephthalic acid was 19.2 g., which is 24.2% of the theoretical yield.

Example 45 150 g. of a mixture composed of equal parts of dipotassium phthalate and dipotassium isophthalate were heated in a rotary autoclave having a net volume of 1.4 liters, in an atmosphere of carbon dioxide and in the presence of 5 g. of cadmium oxide and 2.5 g. of a cadmium oxide kieselguhr catalyst, for 3 hours at a temperature of 430 C. The pressure of carbon dioxide was maintained at atmospheric pressure throughout the heating period. A reaction product weighing 148 g. was formed, 100 g. of which were treated as in Example 1. The yield of terephthalic acid was 34.7 g., which is 49.8% of the theoretical yield.

Example 46 In the production of phthalic acid anhydride, the raw product is subjected to a distillation. The residue remaining in the distillation flask is a black substance of a tar-like consistency. This residue contains comparatively large amounts (50 to 60%) of phthalic acid anhydride. The saponification number of this residue is 710. 148 g. of this residue were admixed with 138 g. potassium carbonate and 10 g. cadmium oxide and the mixture was heated for 1 hour at a temperature of 430 C., in an atmosphere of carbon dioxide, in a rotary autoclave having a net volume of 1.4 liters. Before heating, the pressure of carbon dioxide in the autoclave was adjusted to 50 atmospheres gauge. During the heating step, this pressure reached a maximum of 156 atmospheres gauge. A reaction product weighing 238 g. was formed, 100 g. of which were treated as described in Example 1. The yield of terephthalic acid was 45.2 g., which is 64.9% of the theoretical yield.

Example 47 152 g. of the distillation residue mentioned in the preceding example were neutralized with the theoretical amount of potassium hydroxide. The resulting solution was then evaporated to dryness and the solid substance resulting therefrom was admixed with 15 g. cadmium oxide. This mixture was subsequently heated in a rotary furnace in an atmosphere of carbon dioxide, at atmospheric pressure, for 1 hour at a temperature of 430 C. A reaction product weighing 260 g. was obtained, 100 g. of which were treated in the manner described in Example 1. The terephthalic acid yield was 32.5 e hic is 50.2% of the theoretical yield.

Example 48 g. of the disodium salt of naphthalene-l,8dicarboxylic acid admixed with 2 g. cadmium tluoride were heated for 6 hours at 450 C. in a rotary autoclave having a net volume of 0.2 liter. At the beginning of the run, carbon dioxide was introduced to a pressure of 50 atmospheres gauge, and the maximum pressure at 450 C. was 140 atmospheres gauge. The reaction product, weighing 30.5 g., was dissolved in hot water and the solution was filtered to remove carbon, naphthalene and catalyst; thereafter, the filtrate was acidified with hydrochloric acid at the boiling point. The precipitated acid was filtered off while hot and extracted three times with 250 cc. portions of boiling ethanol. The pure naphthalene-2,6-dicarboxylie acid thus obtained weighed 6.1 g., corresponding to 24.5% of the theoretical yield.

Exmnple 49 A mixture of benzene polycarboxylic acids was obtained from oxidation products of coal, which contained 39% tricarboxylic acids and 61% tetracarboxylic acids. This mixture was neutralized with potassium hydroxide, and potassium benzoate was added to the aqueous solution of the potassium salts in such a manner that the mixture contained two carboxyl groups per benzene nucleus. The aqueous solution was evaporated and the residue was dried at 150 C. and then admixed with 3% cadmium fluoride. g. of this mixture were heated for one-half hour at 430 C. in a rotary autoclave having a volume of 0.2 liter. At the beginning of the run, carbon dioxide was introduced to a pressure of 40 atmospheres, and at 430 C. the maximum pressure was 144 atmospheres. The reaction product obtained thereby in an amount of 33.9 g. was worked up in the above-described manner and yielded 14.4 g. terephthalic acid. Heating an equal quantity of the starting material for one-half hour at 420 C. and under a carbon dioxide ,7 ressure of 20 atmospheres yielded 11.0 g. terephthalic acid.

Example A mixture of 10.0 g. tripotassinm-trimesitinate, 4.0 g. lithium benzoate, 30.0 g. anhydrous potassium carbonate and 3.0 g. cadmium fluoride was heated in a rotary auto clave having a volume of 200 cc. for 5 hours at 420 C. in a carbon dioxide atmosphere at an initial pressure of 50 atmospheres, in the same manner as described in Example 49. The reaction product was worked up in the usual manner and yielded 5.7 g. terephthalic acid.

Example 5] A round iron vessel 22 cm. long and having a diameter of 9 cm. was provided with a kneading device powered by a powerful motor. The kneading device consisted of a planetary stirring system, to which two stirring blades were fastened, and a scraper contacting the walls of the vessel. The iron container was heated from the outside by a lead bath.

150 gm. of a mixture containing about 90% phthalic acid anhydride and 7% benzoic acid were admixed with 140 g. potassium carbonate and dissolved in a small quantity of water, then filtered and evaporated to dryness. The resulting salt was admixed with 9 g. cadmium oxide and placed in the vessel described above. Thereafter the vessel and its contents were heated for 20 minutes at a temperature of 420 C., while passing carbon dioxide into the vessel and over the reaction mass. A dark gray powder weighing 216 g. was formed, which was dissolved in 1000 cc. water by heating the water to the boiling point. The resulting hot solution was then 16 filtered. The filtrate was acidified with hydrochloric acid, whereby a mixture of organic acids was precipitated. T he precipitate was filtered oil and repeatedly extracted with 500 cc. hot water. The yield of pure terephthalic acid was 84 g., which is 55.47% of the theoretical yield.

When the lithium, cesium and rubidium salts of aromatic polycarboxylic acids are substituted for the potassium and sodium salts in the above examples the same reaction takes place with slightly varying yields depending upon the conditions of the reaction.

in larger scale operations approximately the same re sults are obtained as disclosed in the preceding examples. in such larger scale operation, the arranging of the potassium salt of phthalic acid in the form of thin layers has proved to be especially suitable since the autoclave used or such reaction does not require well sealing stulting boxes for the stirrer shaft, and, thus, can be much more readily closed and tightly sealed than an autoclave provided with a stirring device.

The initial operating pressure for the carbon dioxide or other pressure gas is ordinarily between atmospheric pressure and a pressure of 60 atmospheres gauge. An initial pressure of 50 atmospheres gauge has proved to he especially suitable. digher pressures than 60 atmospheres may, of course, also be used. It has been found, however, that such higher pressures do not effect a suflicicntly high increase in yield to warrant the use of autoclaves which are resistant to the ensuing pressures. A rearrangement temperature of about 400 C. and a rearrangement duration between about 6 hours and about 3 hours at such a temperature yields satisfactory results. The lower the temperature the more time is required to effect rearrange ment. Duration and temperature are interdependent from each other. With each starting material and catalyst the best reaction conditions may readily be determined by preliminary experiments carried out in the manner described in the preceding examples.

The potassium salts in general produce better yields than the other alkali metal salts, however, the sodium and other alkali metal salts under favorable reaction conditions and in the presence of catalysts produce commercially attractive yields of tcrephthalic acid. The dirubidium salt results in excellent yields but is not commercially attractive because of its high cost.

Carbon dioxide has proved to be the best protective gas. Under certain conditions nitrogen, carbon monoxidc, ammonia gas, methane, ethane, propane, benzene and other hydrocarbon gases may also be used; lil: .vise the noble gases, although the use of said latter gases is restricted on account of their high cost. Hydrogen should not be used and oxygen, of course, must not be present in the reaction chamber.

Metal oxides have, in general, a favorable effect upon the rearrangement reaction. Especially suitable are lead oxides, iron oxides, titanium dioxide, zirconium dioxide and antimony trioxide although other oxides also cause a substantial increase in yield.

While I have described several examples of the practice of my invention, many changes and variations in the proportions of the reaction components, the kinds and amounts of catalysts added, the reaction temperature, duration and pressure, the methods of working up the reaction mixture and of separating therefrom and purifying terephthalic acid may be made by those skilled in the art in accordance with the principles of my invention and within the scope of the claims annexed hereto.

Any theories as to the nature and course of the rcae tion's described herein are stated for the purpose o1": illustration and without any intention to be bound by the correctness or lack of correctness of any such theories.

I claim:

1. In a process of converting compounds of the group consisting of phthalic acid and isophthalic acid into terephthalic acid, the step of heating a dialkali metal salt of a compound of the group consisting of phthalic acid 17 and isophthalic acid to a temperature between 340 C. and 500 C, in a substantially oxygen-free inert atmosphere, thereby causing rearrangement of a portion of the phthalic acid radicals of said salts into terephthalic acid radicals.

2. In a process of converting compounds of the group consisting of phthalic acid and isophthalic acid into terephthalic acid, the step of heating an alkali metal salt of a compound of the group consisting of phthalic acid and isophthalic acid in a substantially oxygen-free inert atmosphere of carbon dioxide to a temperature between 340 C. and 500 C. and under superatmospheric pressure to cause rearrangement of the phthalic acid radicals of said salts to terephthalic acid radicals.

3. In a process of converting compounds of the group consisting of phthalic acid and isophthalic acid to terephthalic acid, the steps of heating a potassium salt of a compound of the group consisting of phthalic acid and isophthalic acid to a temperature between 340 C. and 500 C. in a substantially oxygen-free inert atmosphere under superatmospheric pressure to cause rearrangement of a portion of the phthalic acid radicals of said salts into terephthalic acid radicals.

4. In a process of converting compounds of the group consisting of phthalic acid and isophthalic acid to terephthalic acid, the steps of heating an alkali metal salt of a compound of said group in a substantially oxygenfree carbon dioxide atmosphere under a pressure between about 30 atmospheres gauge and about 280 atmospheres gauge to a temperature between about 340 C. and 500 C. for 1 to 8 hours while thoroughly agitating the reaction mixture, dissolving the reaction mixture in water, precipitating the carboxylic acids from the resulting aqueous solution by the addition of mineral acid, filtering off the precipitated carboxylic acids, heating said carboxylic acids in water and filtering ofi the undissolved terephthalic acid from the mother liquor.

5. in a process of converting compounds of the group consisting of phthalic acid and isophthalic acid to terephthalic acid, the steps of heating an alkali metal salt of a compound of said group in a substantially oxygen-free inert atmosphere under a pressure between about 30 atmospheres gauge and about 280 atmospheres gauge to a temperature between about 340 C. and 500 C. for 1 to 8 hours while thoroughly agitating the reaction mixture, dissolving the reaction mixture in Water, precipitating the carboxylic acids from the resulting aqueous solution by the addition of mineral acid, filtering off the precipitated carboxylic acids, heating said carboxylic acids in water and recovering the unconverted carboxylic acids from said mother liquor.

6. In a process of converting compounds of the group consisting of phthalic acid and isophthalic acid to terephthalic acid according to claim 4, wherein the dialkali metal salt of said group is formed from a mixture of a compound from said group and potassium carbonate, said mixture, under said rearrangement conditions, forming the potassium salt of the compound from said group.

7. In a process of converting compounds of the group consisting of phthalic acid and isophthalic acid to terephthalic acid according to claim 4, wherein the potassium salt of an acid from said group is heated in the presence of catalysts selected from the group consisting of oxides of the metals lead, iron, titanium, zirconium and antimony.

8. In a process of converting a compound of the group consisting of phthalic acid and isophthalic acid to terephthalic acid, the steps of heating an alkali metal salt of a compound of said group in a substantially oxygen-free,

t8 inert atmosphere to a temperature between 340 C. and 500 C. until rearrangement of the acid radical of said salt to a terephthalic acid raiical takes place, separating terephthalic acid from the rearrangement mixture by acidifying said mixture, separating the precipitated carboxylic acids from the mother liquor, and extracting said precipitated acids by means of boiling water to dissolve therefrom the soluble carboxylic acids.

9. In a process of producing salts of terephthalic acid, the step of heating an alkali metal salt of a compound from the group consisting of phthalic acid and isophthalic acid in a substantially oxygen-free inert atmosphere to temperatures above about 340 C. and not substantially in excess of 500 C. to cause production of alkali metal terephthalate therefrom.

10. The method of producing terephthalic acid from a compound from the group consisting of phthalic acid, isophthalic acid and anhydrides thereof, which comprises converting the starting compound into an alkali metal salt thereof, drying said salt and heating the dry salt in an inert atmosphere substantially free of oxygen to a temperature between about 340 C. and below the decomposition temperature of the said salt and the reaction products until a substantial amount of dialkali metal terephthalate has been produced and converting the dialkali metal terephthalate into terephthalic acid and separating the terephthalic acid from the reaction mixture.

11. In a process of converting phthalic acid into terephthalic acid, the step of heating the di-potassium salt of phthalic acid to a temperature between 340 C. and 500 C., in a substantially oxygen-free inert atmosphere, thereby causing rearrangement of a portion of the phthalic acid radicals into terephthalic acid radicals.

12. In a process of converting phthalic acid to terephthalic acid, the step of heating a potassium salt of phthalic acid in a substantially oxygen-free inert atmosphere of carbon dioxide to a temperature between 340 C. and 500 C. and under superatmospheric pressure to cause rearrangement of the phthalic acid radicals to terephthalic acid radicals.

13. The method of producing terephthalic acid from phthalic acid, which comprises converting said phthalic acid into potassium salt thereof, heating said salt in a substantially dry state and in a substantially oxygenfree inertatmosphere to a temperature between about 340 C. and not substantially above 500 C. until a substantial amount of di-potassium salt of terephthalic acid has been produced, converting said di-potassium salt of terephthalic acid and unreacted potassium phthalate into terephthalic acid and phthalic acid and separating the terephthalic acid from the phthalic acid.

14. The method of producing terephthalic acid from phthalic acid, which comprises converting the phthalic acid into potassium phthalate, drying the potassium phthalate and heating the dry potassium phthalate in an inert atmosphere substantially free of oxygen to a temperature between about 340 C. and below the decomposition temperature of potassium phthalate until a substantial amount of di-potassium terephthalate has been produced and converting the di-potassium terephthalate into terephthalic acid and separating the terephthalic acid from the remainder of the reaction mixture.

References Cited in the file of this patent UNITED STATES PATENTS 1,937,477 Mills et al. Nov. 28, 1933 FOREIGN PATENTS 522,829 Belgium Oct. 15, 1953 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 2,863,914 December 9, 1958 Bernhard Raecke It is hereby certified that error appears in the-printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

for "product '0 9;." read product 30 g,

Column '7, line Signed and eea .ed this 15th day of December 1959.

(SEAL) Attest:

KARL H, AXLINE ROBERT C. WATSON Commissioner of Patents Att esting Officer 

1. IN A PROCESS OF CONVERTING COMPOUNDS OF THE GROUP CONSISTING OF PHTHALIC ACID AND ISOPHTHALIC ACID INTO TEREPHTHALIC ACID, THE STEP OF HEATING A DIALKALI METAL SALT OF A COMPOUND OF THE GROUP CONSISTING OF PHTHALIC ACID AND ISOPHTHALIC ACID TO A TEMPERATURE BETWEEN 340*C. AND 500*C., IN A SUBSTANTIALLY OXYGEN-FREE INERT ATMOSPHERE, THEREBY CAUSING REARRANGEMENT OF A PORTION OF THE PHTHALIC ACID RADICALS OF SAID SALTS INTO TEREPHTHALIC ACID RADICALS. 